1. Field of the Invention
The present invention relates to a current limiting circuit and a method of manufacturing such a current limiting circuit, and more particularly to a current limiting circuit capable of preventing an overcurrent from flowing when a load thereof is short-circuited, and a method of manufacturing such a current limiting circuit.
2. Description of the Related Art
As shown in FIG. 1 of the accompanying drawings, a conventional current limiting circuit comprises an output N-channel vertical MOS transistor (hereinafter referred to as an "output VDMOS transistor") 11 having a gate connected to an input terminal 16, a source connected to ground, and a drain which is supplied with a power supply potential VDD through a load 17, an N-channel MOS transistor (hereinafter referred to as an "NMOS transistor") 13 having a gate connected to the drain of the output VDMOS transistor 11 and a source connected to ground, and three diodes 15 connected in forward direction between the drain of the NMOS transistor 13 and the gate of the output VDMOS transistor 11.
When an input signal is applied to the input terminal 16, the output VDMOS transistor 11 is rendered conductive. If an overcurrent flows through the output VDMOS transistor 11 due to a short-circuit across the load, for example, while the output VDMOS transistor 11 is conductive, the voltage between the drain and source of the output VDMOS transistor 11 is increased, rendering the NMOS transistor 13 conductive. When the NMOS transistor 13 is conductive, the voltage between the gate and source of the output VDMOS transistor 11 is equalized to the sum of the forward voltages across the three diodes 15 and the voltage across the NMOS transistor 13 as it is turned on. Since the output VDMOS transistor 11 operates in a saturated region, it can limit the current flowing therethrough to a certain constant level.
In the illustrated conventional current limiting circuit, the clamping voltage between the gate and source of the output VDMOS transistor 11 is established by a multiple of the forward voltage of each diode 15. Consequently, the clamping voltage can only be selected in steps of about 0.6 V, which is the forward voltage of a general diode, and hence the clamping voltage cannot be set to a desired value more accurately.
Furthermore, inasmuch as the forward voltage of a diode varies greatly as the temperature varies, the clamping voltage tends to shift from the selected value depending on the temperature.